Biscuit isn't afraid of dogs but he is not fond of dogs that run in and out around him. I just found out that his former owner's dog used to chase them in the pasture
well, dang...that explained his dislike of my buddy's dog Bella!
There is lots of ways to get them used to dogs if they are never around them - but I think a dog just barking at a horse is one thing - if a dog ran threateningly at a horse might be a total different ball of wax. I think several people suggestion of "chasing" the dog would probably work for almost any but a vicious dog. LOL I have to work at Biscuit charging Bella when she starts to get on his last nerve and I can tell when he has ran out of "Bella" patience. He has kicked at her before too. So while he is not afraid of her, I know he would love to feel her blood on his hooves!
I did a bomb proof clinic with Biscuit last October and it really helped with getting Biscuit and me on the same page. I learned to not let him evade something he didn't want to do. Focusing on one obstacle at a time helped. Exposure to different noises, "strange" alien things popping up helped me as much as Biscuit!